When I was working in sales this is what they taught me to psychologically trick people into buying whatever shit we were selling. Strap in, this could be long.
First up, everything I learnt in sales worked through what they called 'impulse' selling, which means playing on people's tendencies to make a decision based on their current state of emotion. Salesmen will build your level of 'impulse', and then 'close' you. The 'close' is the point at which they seal the deal, and you give them your money in exchange for whatever they have convinced you that you need.
There are five basic ways that salesmen will 'impulse' you. The acronym they taught us was G.I.F.T.S.
The first was 'Greed'. People are naturally greedy. By which I mean they want more for their money. They want a good deal. If people think they can make or save money, they are more inclined to buy. An example of this is basic 'half price' or 'buy X, get Y free' sales.
I stands for 'Indifference'. People can smell desperation. If they sense that you have a motive for wanting them to do something (like buy) they will be more wary, and want to know your reasons. Therefore, a salesman will try to make it seem as though they do not care whether or not you buy (even if they are on commission). After all, they are only offering you this amazing deal for your own benefit.. They have nothing to gain..
Third was 'Fear of Loss'. Causing people to worry that they will miss out if they don't buy. This can be exploited by making people think that this is their one and only opportunity to purchase at a 'reduced rate', or used in conjunction with 'Greed', for example 'buy in the next 60 minutes and get X free!'.
T, 'The Jones' Theory'. If your community is getting on-board with an idea, there is no reason that you shouldn't too. It's safe. 'It's all the rage'. 'Everybody's doing it'. 'Don't miss out'. This also ties in with 'Fear of Loss'.
The last one is 'Sense of Urgency'. Can be used in similar ways as 'Fear of Loss', i.e. 'buy in the next 60 minutes or else X', or as subtly as a salesman saying that they have other appointments and won't be able to come back and offer you this deal for a too-long period of time. A sense of urgency causes people to buy more impulsively, especially when coupled with a fear of loss.
Once salesmen have 'impulsed' you enough, they will try to 'close' you. I was also taught a number techniques to 'close'.
The first was the 'assumptive close'. This is basically assuming that the person will buy and filling out the paperwork. A common example of this is a salesman simply asking for your your name, and the proceeding with the sale. They will fill out an entire form and then just ask you to sign at the end.
This is often assisted by the 'trial close', where a salesman will slowly push you over the line, while at the same time testing you to see if you are 'impulsed' enough to buy. They will do this by asking you closed questions, aimed at steering you down a conversational track which leads to a sale. Charity workers do this a lot when they ask 'Do you like dolphins?' (yes), 'Do you think dolphin's habitats should be protected?' (yes), 'How much do you spend on beer / tea / coffee a week?' ($5-$50), 'Do think you could put $X towards saving the dolphins?' (umm, well, I guess you got me there..)
Another powerful close is the 'alternative close', where salesmen will offer you one of two choices, both of which result in a sale. 'So would you like the regular option or the slique-deluxe?'. Often presented assumptively (see 'assumptive close').
The last was the 'silent close'. Harder to use, but effective with indecisive buyers or people that pull back when pressured. Basically presenting the overwhelming positives with the easily countered negatives, and then shifting control of the conversation to the buyer, and forcing them to say 'yes' or 'no'. Obviously, the salesman has presented the information in such a way that you would be stupid to say 'no'. After building tension and excitement for the product, they let you come to the decision themselves.
Almost every person who sells goods or services has been taught something along these lines, and the most successful salesmen have this information at the forefront of their minds when they are selling to you. Never forget it. These people just want your money, they honestly do not generally care what you get out of it.